Steerable three-dimensional magnetic field sensor system for detection and classification of metal targets

ABSTRACT

A steerable electromagnetic induction (EMI) sensor system for measuring the magnetic polarizability tensor of a metal target. Instead of creating a vertical magnetic field from a horizontal loop transmitter configuration used by most prior art EMI metal detectors, the transmitter geometry of the sensor system&#39;s antenna is designed especially for creating multiple horizontal and vertical magnetic fields and for steering the same in all directions. The horizontal magnetic field (HMF) antenna has the potential advantage of a relatively uniform magnetic field over a large volume. A second potential advantage of the HMF antenna is that compared to a conventional loop antenna, the magnetic field intensity falls off slowly with distance from the plane of the antenna. Combining two HMF sensor systems creates a steerable two-dimensional magnetic field sensor. Combining the steerable HMF sensor with a vertical magnetic field antenna forms a three-dimensional steerable magnetic field sensor system.

This application claims the benefit of Provisional Application No.60/201,020, filed May 2, 2000.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to detecting and identifying metal targetsin general and, more particularly, to a steerable three-dimensionalmagnetic field sensor system and method for detecting and identifyingmetal targets, such as unexploded ordnance (UXO), underground utilities,high metal content landmines and low metal content landmines buried inthe soil (or visually obscured) based on the electromagnetic response ofthe target to a time-domain wide bandwidth electromagnetic spectrum.

2. Description of the Related Art

Most electromagnetic induction (EMI) metal detectors use a loop antennato create a magnetic field in the vicinity of a metal target for thepurposes of detection and identification. One of the most importantfunctions of a magnetic field antenna is to project a strong magneticfield at the site of the target.

Typical loop antennas are formed of multiple turns of wire around acentral axis. The magnetic field strength of a loop antenna is a strongfunction of distance from the antenna. Far from the antenna, along theaxis of the loop antenna, the field strength varies approximately as1/r³, where r is the distance from the plane of the loop to the object,Off-axis, the antenna field strength and direction tends to be a verycomplex function of position, with the field intensity very strong nearthe wires in the loop and weaker near the center of the loop.

One of the consequences of the loop antenna's complex spatial fieldstrength is the fact that a metal target is excited with a complexmagnetic field. When a buried target of unknown depth is scanned with anEMI sensor, the spatial distribution of the excitation field at thetarget is not known. Some target identification algorithms assume thatthe target is excited with a uniform field. If the field is in factcomplex, the target's time or frequency response to the field is notwell characterized. This may tend to complicate or confound a targetidentification algorithm.

In addition, with the target at the center of the loop, the loopmagnetic field antenna only measures the vertical component of atarget's decay response.

A metal target can be modeled by defining a magnetic polarizabilitytensor:

$\overset{\_}{M} = \begin{pmatrix}{M_{x}(t)} & 0 & 0 \\0 & {M_{y}(t)} & 0 \\0 & 0 & {M_{z}(t)}\end{pmatrix}$where the diagonal components of the tensor are the time responses ofthe target to excitations in an orthogonal reference frame centered onthe target. Models of this nature generally assume that the excitationfield strength is uniform over the target's volume. For a loop antennaoriented directly over a target, the antenna only excites the verticalcomponent of the target's time decay response, M_(z)(t). For accuratetarget classification, it is necessary to measure all three componentsof a target's magnetic polarizability tensor.

Accordingly, a need exists to develop a magnetic field sensor systemthat can project a strong magnetic field deeply into the ground; excitethe target with a uniform magnetic field; and measure thethree-dimensional components of the target's magnetic polarizabilitytensor. As noted above, prior art EMI metal detectors that use loopmagnetic field antennas do not address all of these issues.

SUMMARY OF THE INVENTION

The present invention provides a steerable three-dimensional (3-D)magnetic field sensor system for detection and classification of hiddenor obscured metal targets, as well as voids in soil. The steerable 3-Dmagnetic field sensor system measures the horizontal and verticalcomponents of a metal target's eddy current time decay signature.Instead of creating a vertical magnetic field from a horizontal looptransmitter configuration used by most prior art EMI metal detectors,the transmitter geometry of the sensor system's antenna is designed forcreating horizontal and vertical magnetic fields and for steering thesame. Two horizontal magnetic field (HMF) antennas and a vertical loopelectromagnetic field antenna are combined to form the steerable 3-Dmagnetic field sensor system.

One of the potential advantages of the steerable 3-D magnetic fieldsensor system is the relatively uniform magnetic field that is createdover a large volume by the HMF antennas. A second potential advantage ofthe EMI sensor system is that compared to a conventional loop antenna,the magnetic field intensity falls off slowly with distance from theplane of the antenna. These two advantages potentially make thesteerable 3-D magnetic field sensor system well suited for detection andclassification of metal targets buried deeply in the ground (e.g.,landmines, unexploded ordnance (UXO) and underground utilities).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating the antenna geometry of a horizontalmagnetic field (HMF) antenna of the steerable electromagnetic induction(EMI) sensor system according to the present invention;

FIG. 2 is a chart showing Bx as a function of x at the center of the HMFantenna for different distances from the plane of the HMF antenna in thez direction;

FIG. 3 is a chart showing the angle of Bx as a function of x fordifferent heights above the plane of the HMF antenna;

FIG. 4 is a log-linear chart that compares Bx from HMF antenna accordingto the present invention to Bz of a prior art loop antenna versusdistance from the plane of the HMF antenna;

FIG. 5 illustrates a three-dimensional plot of Bx surrounding the HMFantenna of the present invention;

FIGS. 6A and 6B illustrate the field distribution from the prior artloop antenna and the HMF antenna of the present invention, respectively;

FIG. 7 is a block diagram of the HMF antenna according to the presentinvention;

FIG. 8 is a diagram of an experimental setup for performing test targetmeasurements using the HMF antenna of the present invention;

FIG. 9 is a diagram illustrating the HMF antenna having twisted returnwires, twisted parallel wires and damping resistors;

FIG. 10 is a chart showing time decay response data from a calibrationtest over a time period;

FIG. 11 is a log-log chart of data from an aluminum soda can and a Val59 metal anti-personnel (AP) mine taken approximately 15 cm above theHMF antenna;

FIG. 12A is a diagram illustrating operation of the HMF antenna wherereceiver units are vertically oriented;

FIG. 12B is a diagram illustrating a vertical receiver arrayconfiguration for the receiver units of FIG. 12A;

FIG. 13A is a diagram illustrating operation of the HMF antenna wherereceiver units are horizontally oriented;

FIG. 13B is a diagram illustrating a horizontal receiver arrayconfiguration for the receiver units of FIG. 13A;

FIG. 14 is a diagram illustrating the horizontal receiver arrayconfiguration, the HMF antenna and a differential amplifier;

FIG. 15A is a diagram illustrating detection of a metal target using theHMF antenna and the horizontal receiver array configuration;

FIG. 15B is a diagram illustrating detection of an underground voidusing the HMF antenna and the horizontal receiver array configuration;

FIG. 16 is a diagram illustrating two HMF antennas at right angles toeach other forming a two-dimensional HMF antenna that can generate ahorizontal magnetic field which can be steered in any direction;

FIG. 17 is a diagram illustrating a steerable magnetic field sensorsystem having the two-dimensional HMF antenna; and

FIG. 18 is a diagram illustrating a steerable magnetic field sensorsystem having the three-dimensional HMF antenna.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A discussion is first made as to the underlying physics utilized by ahorizontal magnetic field (HMF) sensor system of the present invention.Following this discussion, a description is provided of the steerabletwo-dimensional HMF sensor system followed by a discussion ofexperimental data demonstrating the sensor system's capabilities.Following this discussion, a description is provided of the steerable3-D magnetic field sensor system which includes two HMF antennas and avertical loop electromagnetic antenna.

I. HMF Antenna Model

The innovative invention of the HMF antenna has not been previouslydiscussed as has the vertical magnetic field antenna. As such, it isinstructive to review the basic physics of the innovative HMF antenna sothat the advantages of such a magnetic field antenna and magnetic fieldreceivers can be appreciated.

Reviewing a basic physics textbook, one of the first geometries astudent is asked to solve is the “sheet current” problem. The textbookproblem and its solution very clearly describes the present antenna'sconfiguration. The problem: “Long, straight conductors with squarecross-section and each carrying current I are laid side by side to forman infinite current sheet. The conductors lie in the xy-plane, areparallel to the y-axis, and carry current in the +y direction. There aren conductors per meter of length measured long the x-axis.” For aninfinite conducting sheet, the field is in the x direction and there isno magnetic field variation in the z direction; the field is constantand is given by:B=μ ₀ n I/2  (1)where I is the current in the wire and n is the number of wires permeter of length measured along the x-axis.

Expressed another way, the sheet current is a horizontal magnetic field(HMF) generator or antenna. The important feature of Equation (1) is thefact that the magnetic field is constant with z, the distance from theplane of the HMF antenna. Equation (1) forms the basis of the presentmagnetic field antenna. The objective is to create an approximation toan infinite sheet current and the magnetic field will have a relativelyuniform shape and a slow magnetic intensity fall-off with distance fromthe plane of the HMF antenna. Additionally, the unique character of thisHMF allows the present invention to use unique magnetic field receiverconfigurations that enhance the time-domain performance of the sensorsystem compared to a conventional loop EMI sensor system.

Preliminary HMF antenna modeling uses the simplified geometry of FIG. 1.Using the Biot-Savart Law, the approximate x and z components of themagnetic field can be written as:

$\begin{matrix}{{{B_{x} = {\frac{\mu_{0}I\mspace{11mu}{z0}}{4\pi}{\sum\limits_{n = 0}^{N}\left( {\left\lbrack {{z0}^{2} + \left( {{x0} - {n\;\Delta\; x}} \right)^{2}} \right\rbrack^{- 1}\left\lbrack {\frac{\left( {L - {y0}} \right)}{\sqrt{\left( {L - {y0}} \right)^{2} + {z0}^{2} + \left( {{x0} - {n\;\Delta\; x}} \right)^{2}}} + \frac{y0}{\sqrt{\left( {{y0}^{2} + {z0}^{2} + \left( {{x0} - {n\;\Delta\; x}} \right)^{2}} \right.}}} \right\rbrack} \right)}}}{B_{z} = {\frac{\mu_{0}I}{4\pi}{\sum\limits_{n = 0}^{N}\left( {{\left( {{x0} - {n\;\Delta\; x}} \right)\left\lbrack {{z0}^{2} + \left( {{x0} - {n\;\Delta\; x}} \right)^{2}} \right\rbrack}^{- 1}\mspace{214mu}\mspace{40mu}\left\lbrack {\frac{\left( {L - {y0}} \right)}{\sqrt{\left( {L - {y0}} \right)^{2} + {z0}^{2} + \left( {{x0} - {n\;\Delta\; x}} \right)^{2}}} + \frac{y0}{\sqrt{\left( {{y0}^{2} + {z0}^{2} + \left( {{x0} - {n\;\Delta\; x}} \right)^{2}} \right.}}} \right\rbrack} \right)}}}}\;} & (2)\end{matrix}$where N is the number of current elements (wires) carrying current I, Δxis the wire separation, L is the length of the HMF antenna, and X0, Y0and Z0 are the location of the magnetic field measurement points inspace. For this preliminary modeling effort, the return path of thecurrent is ignored. The current return path is important and whenconstructing the actual HMF antenna, the return current path can beplaced at a relatively large distance to the nominal detection area. Thetrue Bx component of the magnetic field will be slightly distorted fromthe values generated by the present calculations. The magnetic fielddistorting caused by the actual return path of the wires can becontrolled for the desired field uniformity by moving the wires far fromthe detection area or by adding magnetic shielding to the return pathwires. In any case, the magnetic field distortions are an order ofmagnitude smaller than the spatial field distortion of a loop antenna.In addition, with the simple geometry of a plane set of wires, there isno By component of the magnetic field.

Calculations using Equation (2) were made with the following antennaparameters: X=100 cm, L=300 cm, N=50 and Δx=2 cm. These parameters wereselected for a conceptual application of a HMF sensor mounted on a UXOsurvey cart similar to the United States Navy's Multi-sensor Towed AwayDetection System (MTADS). MTADS uses three EM metal detectors, with 1 mdiameter loop antennas to cover a 3 meter wide search area. The MTADS isdesigned to search and identify buried UXO.

FIG. 2 is a plot of Bx versus x at the center of the HMF antenna (y=150cm) for different distances from the plane of the HMF antenna in the zdirection. Note that the field intensity is relatively uniform exceptclose to the edge of the HMF antenna. For a particular applicationdepth, the HMF antenna parameters can be adjusted for the desired Bxfield uniformity.

FIG. 3 shows the angle of Bx as a function of x (cross antenna track).It is noted that if a receiver coil is placed at the center of the HMFantenna in the plane of the HMF antenna, there is no net flux throughthe receiver coil. The Bz components of the magnetic field cancel. Thisimplies that a horizontal receiver coil so placed will, to first order,not “see” the turn-off transients of a pulsed time-domain version of thesensor. This also implies that a HMF antenna could potentially be usedin a frequency domain sensor system, since the coupling between thetransmitter and receiver are minimized.

FIG. 4 is a log-linear plot that compares Bx from the HMF antenna (1 mby 3 m) to Bz of a prior art loop antenna (1 m diameter) versus distancefrom the plane of the HMF antenna of the present invention. The magneticfields from each antenna have been normalized to 1 at a depth of 10 cmto show the relative field intensity fall-off with distance. Thecalculations were made along the centerline of each antenna.

Over the depth range of 10 cm to 500 cm, FIG. 4 shows that the HMFantenna field strength varies by approximately a factor of 30, whileover the same distance range, the loop antenna varies by a factor of1000. Also shown in FIG. 4 is a third curve of a HMF antenna with areturn current path 1 m away from the primary antenna surface. The Bxfield strength is lower than the HMF field without the return path, butthe field still falls off more slowly than the loop antenna. Over thedistance range of 10 cm to 500 cm, the HMF antenna field strength with areturn path included varies by approximately a factor of 60. This isstill much less than the prior art loop antenna value of 1000.Increasing the return path separation distance or using some form ofmagnetic shielding will reduce the return path effect even more.

FIG. 5 shows a three-dimensional (3-D) plot of Bx surrounding the HMFantenna. Note that the field in both the x and y directions isrelatively uniform, except near the current carrying wires and edges ofthe HMF antenna. As one would expect, the field near the wires is veryintense, causing local “hot spots.”

Another way to view the field distribution from the loop and HMFantennas is shown in FIGS. 6A and 6B. Here, we have a simplifiedconceptual diagram of the magnetic field vectors from the differentantennas. FIG. 6A shows the magnetic field surrounding a simple loopantenna looking from the side. The field is relatively uniform in thecenter of the antenna and is oriented in the vertical direction. As wemove away from the plane of the loop in the −z direction, the fieldcontinues to be vertical along the axis, but off-axis the field has amore complex shape. The field is approximately horizontal under the loopconductors.

FIG. 6B shows the magnetic field surrounding the HMF antenna lookingdown the current carrying wires. The current carrying wires areperpendicular to the plane of the paper and the current direction is outof the paper. In the region near the center of the HMF antenna, themagnetic field is in the horizontal direction. As one moves from thecenter of the HMF antenna, the field becomes more complex, with Bzstarting to dominate at the edge. However, as FIGS. 2 and 5 show, thefield remains relatively uniform in the x direction.

II. Sensor System Description

The steerable 3-D magnetic field sensor system is composed of threesubsystem components: (1) two identical time-domain HMF electromagneticsensor subsystems, each producing a HMF at right angles to each other;and (2) a conventional horizontal loop antenna magnetic field sensorsystem that produces a vertical magnetic field. Each HMF subsystem inturn is composed of two basic components: (1) the HMF antenna and (2)the magnetic field receiver.

A magnetic field is a vector quantity. It has magnitude and directionand can be decomposed into individual components in an orthogonalcoordinate system. For example, in a Cartesian coordinate system of X, Yand Z, a magnetic field vector can be represented by Bx, By and Bz. Ifwe generate in the same volume (spatial region) a magnetic field in theX direction, Bx; generate a magnetic field in the Y direction, By; andgenerate a magnetic field in the Z direction, Bz, the individualmagnetic field components combine to form a new magnetic field that hasa magnitude and direction given by:

${\overset{->}{B}} = \sqrt{B_{x}^{2} + B_{y}^{2} + B_{Z}^{2}}$$\theta = {\tan^{- 1}\left\lbrack \frac{B_{y}}{B_{X}} \right\rbrack}$$\delta = {\tan^{- 1}\left\lbrack \frac{B_{z}}{\sqrt{B_{x}^{2} + B_{y}^{2}}} \right\rbrack}$where θ is the angle measured from the x axis in the XY plane and δ isthe angle from the XY plane to the B vector.

By varying the magnitude of the three magnetic field components, amagnetic field can be projected in any direction. A X direction magneticfield is generated by a X directed HMF antenna, a Y direction magneticfield is generated by a Y directed HMF antenna, and a Z directionmagnetic field is generated by a Z directed horizontal loop antenna.Combining all three antennas, a 3-D magnetic field sensor system of thepresent invention is created.

The details of the HMF subsystem is described first. Later, the use oftwo HMF subsystems and the vertical magnetic field sensor (horizontalloop) will be described that show the creation of the steerable 3-Dmagnetic field sensor system.

FIG. 7 is a simplified block diagram of the HMF sensor system designatedby reference numeral 100 according to the present invention. Anapproximation to a sheet current is created by a series of closelyspaced parallel wires 102. These wires form the active surface of theHMF antenna 104. The closely spaced parallel wires 102 are connected tohigh-speed electronic switches (E-Switch) 106.

Return current wire segments 108 of wires 102 exit the E-Switches 106 ina direction perpendicular to the plane of the HMF antenna 104. Thecircuit is completed with a set of parallel return wire segments 110relatively far away from the active surface of the HMF antenna 104. Asnoted above, the return wire segments 110 of wires 102 complicate theexact magnetic field geometry of the HMF antenna 104, but close to thecenter of the active surface of the HMF antenna 104, the horizontalcomponent of the magnetic field dominates the other components.

A preferred embodiment of the sensor system is described. The current inthe HMF antenna 104 is controlled by the high speed E-Switches 106operating in parallel. The E-Switches 106 are electronic relays withvery fast turn-off times, preferably constructed using insulated gate,bipolar transistors in a floating configuration. One skilled in the artcan construct the E-Switch 106. Opto-isolators are used to couple theE-Switches 106 to the ground-referenced pulse control circuitry 112.

The HMF magnetic field sensor system 100 is preferably configured sothat a single power supply 114 provides current to the HMF antenna 104.It is understood that a different power supply arrangement could be usedthat satisfied specific application requirements. For example, thepresent embodiment requires a relatively high voltage and high currentpower supply to drive the chain of E-Switches 106. Multiple lowervoltage and/or lower current power supplies could be employed that havethe same desired magnetic field characteristics. Also, the powersupplies could be configured so that different current is flowingthrough different portions of the HMF antenna wires. Using differentcurrent levels in different parts of the HMF antenna 104 allows one totailor the spatial character of the magnetic field.

The E-Switches 106 turn-off the antenna current in less than about 400ns (90% to 10% amplitude). To reduce the number of E-Switches 106, eachE-Switch 106 controls four closely spaced parallel wires 102. It isunderstood that the number of wires controlled by the E-Switches 106 isonly limited by the exact electrical characteristics of the E-Switches106 and the electrical properties of the wires (e.g., resistance,inductance and capacitance). More or less wires could be connected tothe E-Switches 106 depending on the application.

Two banks 116 (see FIG. 8) of 16 E-Switches 106 control 64 parallelwires 102 forming the active surface of the HMF antenna 104. The size ofthe HMF antenna 104 is approximately 80 cm by 180 cm and the parallelwires 102 are spaced about 1.3 cm apart.

FIG. 9 is a diagram illustrating a wiring configuration of the HMFantenna 104 having twisted parallel wires 102, twisted return wires 110and damping resistors 146. FIG. 9 only shows one twisted parallel wire102 and one twisted return wire 102 for clarity. It is to be appreciatedthat all the parallel wires 102 and all return wires 110 are twisted.E-Switches 106 are provided to control the antenna current flow. Whenthe E-Switch 106 goes from a closed position (current flowing in loop)to an open position, a damping resistor 146 placed across the loopdampens the current as quickly as possible without circuit oscillationor a long decay time (critically damped).

The inventive arrangement of wires and damping resistors 146 minimizescircuit oscillation, minimizes the time for current damping andminimizes the magnetic field collapse time. In a time-domain metaldetection sensor, the sooner the primary excitation magnetic fielddisappears, the sooner the small eddy currents from the target can bemeasured. The decay of the primary magnetic field is governed by thedecay of the current in the wire after the switch is opened.

Ignoring capacitive effects, the decay time on the loop is given by L/R,where L is the inductance of the loop and R is the resistance of theloop. Minimizing L and making R as large as possible without causingoscillation minimizes the decay of the current in the loop andtherefore, the presence of the primary excitation magnetic field.Breaking the loop up into segments as shown in FIG. 9 reduces the L ofthe segment. Having a return wire 110 between the two switches 106allows a damping resistor 146 to be placed across the wire segment,therefore, damping the inductance of that wire segment.

FIG. 9 shows two damping resistors 146. Although not necessary for theoperation of the damping function, using two resistors 146 add symmetryto the damping function. In wide-bandwidth circuits, symmetric circuitlayouts work more effectively. The HMF antenna arrangement also workswith just one E-Switch 106 controlling the section of wires, but lesseffectively. Additionally, a single resistor could be used for all thewires controlled by the E-Switch 106.

III. Experimental Results

III.1 Experimental Setup

Experiments were conducted to validate the operation of the HMF sensorsystem. FIG. 8 shows a diagram of the experimental setup (top view) forthe test target measurements. Two 15 cm by 15 cm, 16-turn printedcircuit board receiver coils 120 were placed near the center of the HMFantenna 104. The receiver coils 120 were connected in a differentialarrangement to subtract any residual coupled antenna decay current andfar-field electrical noise.

Test targets 122 were placed directly over one edge of the receiver coil120 as shown in FIG. 8. The receiver signal was amplified by awide-bandwidth, multi-stage differential amplifier (not shown). Theoutput of the amplifier was digitized with a data acquisition systemmounted in a personal computer.

III.2 Target Responses

To validate the sensor system's ability to accurately measure time decayresponses from metal targets, the sensor system 100 was tested with acalibration loop. A thin-wire loop can be modeled with a singleexponential decay parameter that can be calculated analytically fromtheory.

A calibration loop was formed from a single turn of #22AWG (AmericanWire Gage) copper wire with a diameter of 10.1 cm. The loop was placedin three orientations over the sensor system 100: the axis parallel tothe plane of the HMF antenna 104 (i.e., maximum coupling to Bx flux) atz=10 cm; the axis 45 degrees to the plane of the HMF antenna 104 withcenter of loop at z=10 cm; and the axis perpendicular to the plane ofthe HMF antenna 104 with center of loop at z=10 cm.

FIG. 10 shows time decay response data from the calibration test overthe time period of 8 μs to 200 μs. A nonlinear least-squares method wasused to fit the wire loop time decay response data to a singleexponential-term equation over the time range of 10 μs to 50 μs. Thecalculated time decay of the wire loop is 20.4 μs and compares favorablywith the measure time decay of 20.6 μs for both the parallel and 45degree orientations. The results of the time constant calibration givesconfidence that the sensor system 100 measures accurate target timedecay responses.

FIG. 10 shows three features of the target responses as measured by thesensor system 100:

First, while the amplitude of the time decay response may change due tothe different flux coupling between the HMF antenna 104 and receiverunit 120, the time decay does not. The 0° and 45° time decay responsesare the same.

Second, when the loop target axis is perpendicular to the HMF antenna104, the response is approximately zero. When the calibration loop isoriented in this fashion, there is approximately zero flux coupling fromthe HMF antenna 104 into the loop. Therefore, there is no flux change toexcite eddy currents. This result validates the idea that the primarymagnetic field component, at this location over the HMF antenna 104, ishorizontal.

Third, target time decay response can be measured relatively close tothe antenna turn-off time.

The time decay data in the time region between 0 μs and 8 μs is notshown in FIG. 10, since the receiver unit 120 was either in saturationor was oscillating. FIG. 11 shows some of these sensor artifacts in thetime region of 5 μs to 8 μs. It is contemplated to use a higherbandwidth and wider dynamic range magnetic field antenna to measuretarget time decay closer to the antenna turn off time. This would enablethe detection of low metal content targets, since such targets have afast time decay response. Since the HMF antenna turns off in less than 1μs, the performance (i.e., increased bandwidth) of the sensor system canbe improved by using higher frequency receiver coils or higher bandwidthmagnetic sensors.

The primary objective of the HMF sensor system 100 is targetidentification and classification based on eddy current time decay. Withthis objective in mind, FIG. 11 presents preliminary time decay responsedata from two different targets at two different orientations to the HMFantenna plane. The absolute amplitude of the target time decay responsedoes not govern the target characterization process. Therefore, in orderto show more clearly the differences in the relative time decayresponses of different targets with large differences in absoluteamplitude, the time decay data amplitudes have been normalized to 1 at15 μs. FIG. 11 is a log-log plot of data from an aluminum soda can and aVal 59 metal anti-personnel (AP) mine taken approximately 15 cm abovethe plane of the HMF antenna 104. The two targets were oriented bothvertically and horizontally relative to the plane of the HMF antenna104. FIG. 11 clearly shows that the two targets can be differentiatedfrom each other based solely on their time decay responses. This targetdifferentiation has been demonstrated in other research and forms thebasis of target identification and classification. FIG. 11 demonstratesthat the sensor system 100 has the capability to measure target timedecay responses that are different for different targets, and foridentifying the different targets.

III.3 Operation

A description will now be provided as to the operation of the HMFantenna 104 with different receiver configurations. These receiverconfigurations can be used singly or in combination. We start withsimple configurations that show the underlying concepts of the HMFsensor system features and move to more complex receiver configurationsthat take full advantage of the HMF antenna characteristics.

FIG. 12A is a side view diagram illustrating operation of the HMFantenna 104 where the receiver units 120 a, 120 b are verticallyoriented. FIG. 9B is a top view diagram illustrating a vertical receiverarray configuration for the receiver units 120 a, 120 b of FIG. 9A.Vertically oriented receivers refers to the axis of sensitivity of thereceiver, i.e., the receiver is sensitive to vertical magnetic fields.The receiver units 120 are located near the opposite edges of the HMFantenna 104 above the plane of the HMF antenna 104 (toward the directionof the metal target 140) and are connected to differential amplifier121. Even though the receiver units 120 are illustrated as using aninduction coil, it is contemplated that the receiver units 120 can useother types of magnetic sensors.

FIG. 12A shows the metal target 140 located near the centerline of theHMF antenna 104 at some distance from the plane of the HMF antenna 104.The HMF antenna 104 generates a magnetic field in the positivex-direction as indicated by the HMF excitation arrow pointing to theright. When the magnetic field is turned off, the metal target 140responds by generating a magnetic field that opposes the collapsingmagnetic field, i.e., eddy currents in the metal target 140 aregenerated. FIG. 12A shows conceptually the magnetic field (targetresponse) generated by the metal target's eddy currents. At the leftreceiver unit 120 a, the magnetic flux from the target response isdownward in the −z direction, (−Bz). The vertical (z sensitive) magneticfield receiver unit 120 a detects this magnetic flux and sends thesignal VI to a differential amplifier's negative input. The differentialamplifier is designated by reference numeral 144.

At the same time, the right receiver unit 120 b intercepts the magneticflux from the target response which is upward, +z direction, (+Bz). Thenominally identical right vertical magnetic field receiver unit 120 bdetects this magnetic flux and sends the signal V₂ to the differentialamplifier's positive input. The output from the differential amplifier144 can be written as:V=V ₂ −V ₁ +N ₂ −N ₁where V₁ is the left receiver unit signal, V₂ is the right receiver unitsignal, N₁ is the electromagnetic (EM) noise (e.g., interference)measured by the left receiver unit 120 a and N₂ is the EM noise measuredby the right receiver unit 120 b. Since the EM noise seen by bothreceiver units 120 a, 120 b is nominally the same and V₁≈−V₂, the outputof the differential amplifier 144 is then,V=2*V ₂The EM noise terms cancel and we double our target signal. Theamplification of the target signal is dependent on the spatialrelationship between the target 140 and the sensor system 100. However,the EM noise cancellation is still effective and allows the sensorsystem 100 to operate in EM noisy environments.

It is also obvious that the receiver units 120 could be wired togetherdirectly to create the same effect as that generated by the differentialamplifier 144. This alternate connection of the receiver units 120 isunderstood in the following discussion. The use of the differentialamplifier 144 makes the discussion of signal addition and subtractionclear.

FIG. 13A is a side view diagram illustrating operation of the HMFantenna where the receiver units 120 a, 120 b are horizontally oriented.FIG. 13B is a top view diagram illustrating a horizontal receiver arrayconfiguration for the receiver units 120 a, 120 b of FIG. 13A. Referringback to FIG. 13A, the receiver units 120 a, 120 b are nominally locatedalong the centerline of the HMF antenna 104. When the target 140 islocated directly under the plane of the HMF antenna 104, the target fluxis primarily horizontal at the plane of the antenna 104. The horizontalreceiver units 120 a, 120 b detect this flux.

FIG. 13A shows the HMF antenna 104 generating a magnetic field in thenegative x-direction as indicated by the HMF excitation arrow pointingto the left. When the magnetic field is turned off, the metal target 140responds by generating a magnetic field that opposes the collapsingmagnetic field, i.e., eddy currents in the metal target 140. FIG. 13Ashows conceptually the magnetic field (target response) generated by themetal target's eddy currents. The target response flux at the plane ofthe HMF antenna 104 is pointing to the left as denoted by the smallarrows. The signals from the two receivers units 120 a, 120 b arepreferably summed in an amplifier 145. The signal measured by thereceiver units 120 a, 120 b can be written as:Signal Output=R1(T)+R1(N)+R1(A)+R2(T)+R2(N)+R2(A).

R1 is the top receiver unit 120 a and R2 is the bottom receiver unit 120b, R1(T) is the top receiver unit's target signal, R1 (N) is the topreceiver unit's EM noise signal, R1(A) is the top receiver unit'santenna signal, R2(T) is the bottom receiver unit's target signal, R2(N)is the bottom receiver unit's EM noise signal and R2(A) is the bottomreceiver unit's antenna signal. If the two receiver units 120 a, 120 bare placed symmetrically on the antenna 104, then R1(A)=−R2(A) and theantenna signals cancel when added together by the summing amplifier 145.One is then left with approximately twice the target signal and twicethe EM noise signal.

FIGS. 14–15B illustrate how to reconfigure the receiver units 120 a, 120b in FIG. 13A for both antenna flux cancellation and additionally, fluxcancellation from EM noise and the eddy currents generated in the ground(ground response). The feature of ground eddy current cancellation iscalled ground-balancing and is an important concept for low-metal targetand underground void detection.

FIG. 14 is a diagram illustrating the horizontal receiver arrayconfiguration having the horizontal receiver units 120 mounted in adifferential mode. Receiver unit 1 is mounted on the top side of the HMFantenna 104 and receiver unit 2 is mounted on the bottom side of the HMFantenna 104 as in FIG. 13A. The signals of the two receiver units 1, 2are summed together and the output is sent to the plus input ofdifferential amplifier 148. Additionally, a second set of identicalreceiver units 3, 4 are located near the first set, but not directlyover the target under study. Their summed output is sent to the negativeinput of the differential amplifier 148. Using similar notation asabove, the output signal of the differential amplifier 148 can bewritten as:A=R1(T)+R1(N)+R1(A)+R1(G)+R2(T)+R2(N)+R2(A)+R2(G)B=R3(N)+R3(A)+R3(G)+R4(N)+R4(A)+R4(G).Where R1 and R3 are the two top receiver units 1, 2 and R2 and R4 arethe two bottom receiver units 3, 4; R1(T), R1(N), R1(A), and R1(G) arethe first receiver unit's target, EM noise, antenna and ground signals,respectively; R2(T), R2(N), R2(A), and R2(G) are the second receiverunit's target, EM noise, antenna, and ground signals, respectively;R3(N), R3(A), and R3(G) are the third receiver unit's EM noise, antenna,and ground signals, respectively; and R4(N), R4(A), and R4(G) are thefourth receiver unit's EM noise, antenna, and ground signals,respectively.

If the top two pairs of receiver units 1, 2 are placed symmetrically onthe antenna 104, then:R1(T)≈R2(T)R1(A)=R3(A)=−R2(A)=−R4(A)R1(N)=R3(N)=R2(N)=R4(N).

The differential amplifier 148 (or alternatively, differencing viadirect connection with wires in reverse order as in the case ofcounter-wound induction coil receivers) subtracts B from A and:Output=A−BOutput=2*R1(T)+R1(G)+R2(G)−R3(G)−R4(G).

FIG. 15A shows the case of a medium or large metal target 140 that has aresponse that is larger than the ground response. The relative size ofthe arrows in FIG. 15A indicate the relative response's from the metaland ground. Then,Output=2*R1(T)R(G)<<R(T).Hence, the medium or large metal target 140 is detected by the system100 and/or an operator.

For the case of a small metal target where the metal response is notlarge with respect to the ground, then,R1(G)≈R3(G)R2(G)≈R4(G)Output≈2*R1(T).

The case of an underground void is shown by FIG. 15B, where theunderground void is designated generally by reference numeral 142. Theground response of the underground void 142 is less than the groundresponse without the underground void 142. When the signals aresubtracted in differential amplifier 151, the void signal is negative.Accordingly, the system 100 and/or the operator detect the existence ofthe underground void 142.

If there is an underground void 142 and a small metal content targetpresent, as is the case for a low-metal content mine, the time decaysignal will be composed of both negative (void) and positive (metal)signals. The eddy current time decay of a void and metal target are verydifferent. Accurately measuring the time decay history of the targetresponse allows one to separate the void and metal signals. Theexistence of a coincident metal and void signal is an indication of alow-metal content mine.

IV. Forming the Steerable 3-D Magnetic Field Sensor System

One method to model a metal target is to define a magneticpolarizability tensor

$\begin{matrix}{\overset{->}{M} = {\begin{pmatrix}{M_{x}(t)} & 0 & 0 \\0 & {M_{y}(t)} & 0 \\0 & 0 & {M_{z}(t)}\end{pmatrix}.}} & (3)\end{matrix}$where the diagonal components of the tensor are the time responses ofthe target to excitations in an orthogonal reference frame centered onthe target. For a loop antenna oriented directly over a target, theantenna only excites the vertical component of the target's time decayresponse. For accurate target classification, it is desirable to measureall three components of a target's magnetic polarizability tensor.Accordingly, a discussion will now be presented for combining two singleHMF sensor systems 100 to form a steerable two-dimensional (2-D) HMFsensor system and then combining a steerable 2-D HMF sensor system witha vertical loop antenna sensor system and forming the steerable 3-Dmagnetic field sensor system.

With reference to FIGS. 16 and 17, there are shown two HMF antennas 104a, 104 b at right angles to each other forming a two-dimensional HMFantenna 150 that can generate a horizontal magnetic field which can besteered in any direction in the plane of the antennas 104 a, 104 b, notjust the direction perpendicular to the current flow in the antennawires 102, and a diagram of a steerable HMF sensor system 200 having thetwo-dimensional HMF antenna 150, respectively. FIG. 16 shows one wire102 a representing x-direction HMF antenna 104 a with current flow Ix inthe x-direction and one wire 102 b representing y-direction HMF antenna104 b with current flow Iy in the y-direction.

By controlling the current separately in each HMF antenna 104 a, 104 busing current control circuitry 160 a, 160 b under computer control 170(FIG. 17), one can create a new magnetic field pointed in any directionin the plane of the HMF antennas 104 a, 104 b. The new magnetic field isgiven by B=(Bx²+By²)^(1/2). The angle of the field is given byθ=tan⁻¹[By/Bx]. It is provided that receiver units 120 are providedadjacent each of the HMF antennas 104 a, 104 b as described above withreference to FIGS. 8 and 12A–15B.

One skilled in the art would appreciate that additional HMF antennas 104may be provided to the system 200.

FIG. 18 is a schematic diagram of the steerable 3-D magnetic fieldsensor system 300 according to the present invention. The sensor system300 adds to the 2-D steerable HMF sensor system 150 a vertical loopantenna 180 which adds the third dimension to the steerable magneticfield sensor system. Accordingly, the generated magnetic field of thesensor system 300 is a summation of the magnetic field in the x-axisdirection generated by the HMF antenna 104 a, the magnetic field in they-axis direction generated by the HMF antenna 104 b, and the magneticfield in the z-axis direction generated by the vertical loop antenna180.

At least one vertical magnetic field receiver (not shown) is understoodto be included in the sensor system 300. A z-antenna current controlcircuitry 160 c is also provided which is under computer control 170 forcontrolling the magnetic field in the z-direction.

It is contemplated to provide a computer data collection system to thesensor system 100 for digitizing the time decay data from the output ofthe receivers 120. The data would then be analyzed to optimize thesystem's operating parameters, such as antenna current, digitizer samplerate and time sampling window, for optimal target data collection andcharacterization. Once a target's time decay response is measured, thetarget can then be classified and identified using a matched filter orother classification/identification approach. It is further contemplatedto configure the steerable magnetic field sensor system 100 for mountingto a vehicle or aircraft to provide a vehicle mounted or airborne minedetector sensor system.

It is further contemplated to configure the HMF antenna 104, thesteerable 2-D or 3-D magnetic field sensor systems for mounting to avehicle to provide a vehicle mounted mine/UXO detector sensor system;for detection, localization and identification of buried utilities; andfor detection and identification of hidden targets at critical points ofentry, such as airports and secured areas. The antenna and systemsconfigured could also be for mounting to an aircraft or other airborneplatform for airborne active electromagnetic surveys for mines, andburied ordnance and utilities.

What has been described herein is merely illustrative of the applicationof the principles of the present invention. For example, the functionsdescribed above and implemented as the best mode for operating thepresent invention are for illustration purposes only. Other arrangementsand methods may be implemented by those skilled in the art withoutdeparting from the scope and spirit of this invention.

1. A sensor system for inducing eddy currents in objects comprising: an antenna configured for generating a magnetic field in a direction perpendicular to current flow in a plurality of wires along a plane of the antenna; at least two receiver units in proximity to the plane of the antenna, each of said two receiver units configured for converting the eddy currents into a first signal and a second signal; and an amplifier for receiving the first and second signals.
 2. The system according to claim 1, wherein the antenna has a rectangular configuration having a length of approximately 180 cm and a width of approximately 80 cm.
 3. The system according to claim 1, wherein adjacent wires of the plurality of wires along the plane of the antenna are equally-spaced and parallel with respect to each other.
 4. The system according to claim 1, wherein the amplifier subtracts the first signal from the second signal.
 5. The system according to claim 1, wherein each of that at least two receiver units measures approximately 15×15 cm and has 16 loop turns.
 6. The system according to claim 1, wherein the at least two receiver units are provided above the plane of the antenna.
 7. The system according to claim 1, wherein the at least two receiver units are provided below the plane of the antenna.
 8. The system according to claim 1, where one of the at least two receiver units is placed below the plane of the antenna and the other of the at least two receiver units is placed above the plane of the antenna.
 9. The system according to claim 1, wherein the amplifier adds the first signal and the second signal.
 10. The system according to claim 1, wherein each of the wires along the plane of the antenna is twisted with an adjacent wire and both wires are connected by at least one damping resistor.
 11. The system according to claim 1, further comprising another antenna oriented at approximately 90 degrees with respect to the antenna for generating another magnetic field in a direction perpendicular to the current flow in a plurality of wires along its plane.
 12. The system according to claim 11, further comprising current control circuitry for controlling the direction of a pulsed magnetic field which is a summation of the magnetic fields generated by the two antennas.
 13. The system according to claim 11, further comprising an antenna encircling the two antennas for generating another magnetic field.
 14. The system according to claim 13, further comprising current control circuitry for controlling the direction of a pulsed magnetic field which is a summation of the magnetic fields generated by the three antennas.
 15. A sensor system for inducing eddy currents in objects comprising: an antenna configured for generating a pulsed magnetic field in a direction perpendicular to current flow in a plurality of wires along a plane of the antenna; at least two receiver units in proximity to the plane of the antenna , each of said two receiver units configured for converting the eddy currents into a first signal and a second signal; and an amplifier for receiving the first and second signals; wherein the antenna includes a first bank and a second bank of switches, the first bank of switches is at one end of the antenna and the second bank of is at an opposite end of the antenna, and the first bank of switches and the second bank of switches are connected by a plurality of return wire loops and the plurality of wires along the plane of the antenna.
 16. The system according to claim 15, wherein the first bank and the second bank of switches are controlled by pulse control circuitry.
 17. The system according to claim 15, wherein each of the plurality of return wire loops includes a wire segment perpendicular to the equally-spaced parallel wires.
 18. The system according to claim 15, wherein the first bank and the second bank of switches each include 16 switches.
 19. The system according to claim 18, wherein four equally-spaced parallel wires of the plurality of wires couple each of the switches of the first bank of switches with a respective switch of the second bank of switches.
 20. The system according to claim 15, wherein each of the plurality of return wire loops include two wires connected by at least one damping resistor, and wherein each of the plurality of return wire loops include a twisted portion.
 21. A sensor system comprising: first, second and third antennas configured for generating respective magnetic fields in three directions perpendicular to each other; wherein the first antenna has a plurality of wires along a plane thereof for generating a magnetic field in a first direction perpendicular to the current flow in the plurality of wires; and control circuitry configured to control the generation of a pulsed magnetic field to produce a magnetic field in selected ones of said three directions, including to selectively produce a magnetic field in the direction perpendicular to the current flow in the plurality of wires; wherein the first antenna includes a first bank and a second bank of switches, the first bank of switches is at one end of the first antenna and the second bank of switches is at an opposite end of the first antenna, and the first bank of switches and the second bank of switches are connected by a plurality of return wire loops and the plurality of wires.
 22. The system according to claim 21, wherein the plurality of return wire loops include two wires connected by at least one damping resistor, and wherein each of the plurality of return wire loops include a twisted portion.
 23. An antenna system for introducing eddy currents in objects comprising: a first antenna configured for generating a pulsed magnetic field in the x-axis direction; a second antenna configured for generating a pulsed magnetic field in the y-axis direction and oriented at a 90-degree angle with respect to the first antenna; and a third antenna configured for generating a pulsed magnetic field in the z-axis direction and encircling the first and second antennas.
 24. The system according to claim 23, further comprising current control circuitry for controlling the direction of a pulsed magnetic field which is a summation of the pulsed magnetic field in the x-axis direction, the pulsed magnetic field in the y-axis direction, and the pulsed magnetic field in the z-axis direction.
 25. The system according to claim 23, wherein the first and second antennas include a plurality of wires, and wherein adjacent wires are connected to each other by at least one damping resistor.
 26. A sensor system comprising: first, second and third antennas configured for generating respective pulsed magnetic fields in three directions perpendicular to each other; wherein the first antenna has a plurality of wires along a plane thereof for generating a pulsed magnetic field in a first direction perpendicular to the current flow in the plurality of wires; and control circuitry configured to control the generation of a pulsed magnetic field to produce a pulsed magnetic field in selected ones of said three directions, including to selectively produce a pulsed magnetic field in the direction perpendicular to the current flow in the plurality of wires.
 27. The system according to claim 26, wherein the plurality of wires are equally-spaced with respect to each other.
 28. The system according to claim 26, wherein each of the plurality of wires is twisted with an adjacent wire and both wires are connected by at least one damping resistor.
 29. The system according to claim 26, wherein the antenna includes a first antenna and a second antenna oriented at approximately 90 degrees with respect to each other, and wherein the pulsed magnetic field is a summation of a magnetic field generated by the first antenna and a magnetic field generated by the second antenna.
 30. The system according to claim 29, wherein the antenna includes a third antenna encircling the first and second antennas, and wherein the pulsed magnetic field is a summation of the pulsed magnetic field generated by the first antenna, the magnetic field generated by the second antenna, and a pulsed magnetic field generated by the third antenna. 